This invention relates to optical fiber units forming high-density optical cables in optical communication networks, and more particularly to the structure of an optical fiber unit comprising a core body having a twisted tensile strength member extended along the central axis thereof and spiral grooves in the outer surface thereof; and tape-shaped optical fibers placed in the spiral grooves of the core body, in which the optical fiber residual strain produced during manufacture can be controlled with high precision.
A variety of research has been conducted on the construction of an optical fiber unit of this type. However, the structure of an optical fiber unit suitable for precisely controlling the residual strain which is produced in optical fibers during the manufacture of the optical fiber unit has not been developed yet.
In the manufacture of the above-described optical fiber unit, during insertion of a plurality of tape-shaped optical fiber assemblies, which comprise multiple optical fibers in the form of a tape (hereinafter referred to as "tape-shaped optical fibers", when applicable), into spiral grooves formed in the outer surface of the core body, in general a predetermined tensile force is applied to the core body to hold the latter straight or to stably feed the core body to the insertion point. When the core body is tensioned in this manner, angular moments around the central axis of the core body are produced in the twisted tensile strength member, and in ribs formed between the spiral grooves of the core body in such a manner that the pitches thereof are increased. However, in the conventional optical fiber unit, no counter measure is taken against these angular moments. Therefore, the conventional optical fiber unit is subjected to twisting of the core body by the aforementioned tensile force, resulting in a change in the pitch of the spiral grooves.
That is, when the pitch p of the spiral grooves is changed to a value p' by the twist of the core body, the spiral line length changes according to the following equation (1): ##EQU1## In general, when the tape-shaped optical fibers are inserted into the spiral grooves of the core body, a supply tensile force T.sub.s for feeding the core body and the supply tensile force T.sub.t for supplying the tape-shaped optical fibers into the spiral grooves are determined according to the following expression (2) so that, when the optical fiber unit is released from the tensile force, the tape-shaped optical fibers are sufficiently small in residual elongation strain: ##EQU2## where E.sub.s and S.sub.s are the equivalent elastic modulus and the sectional area of the core body, respectively, and E.sub.t and S.sub.t are the equivalent elastic modulus and the sectional area of each tape-shaped optical fiber, respectively. However, if the core body is twisted when the tape-shaped optical fibers are inserted into the spiral grooves of the core body, the actual residual strain .epsilon. of the tape-shaped optical fibers is as follows: ##EQU3## That is, it becomes difficult to precisely control the residual strain.